S-130 Weather Observation 5 — Questions and Answers
Question 1: What is the 'mixing height' and how does it influence fire behavior?
- The altitude at which aircraft must mix retardant before dropping
- The altitude up to which the atmosphere is well-mixed, affecting convective column development and smoke transport (Correct answer)
- The depth of the fuel bed that fire mixes through during burning
- The height at which wind direction changes 90 degrees
Correct answer: The altitude up to which the atmosphere is well-mixed, affecting convective column development and smoke transport
Mixing height defines how high convective air can rise; a low mixing height suppresses fire convection while a high mixing height allows strong fire columns and spotting.
Question 2: Which factor best explains why fires in steep canyon terrain can suddenly 'blow up' in the afternoon?
- Cooler temperatures in canyons reduce fuel moisture faster
- Afternoon heating creates strong upslope winds that align with fire spread direction (Correct answer)
- Canyon shade keeps fuels wetter until afternoon when they suddenly dry
- Canyon walls block wind until afternoon when pressure overcomes the barrier
Correct answer: Afternoon heating creates strong upslope winds that align with fire spread direction
Afternoon solar heating intensifies upslope (anabatic) winds in canyons, which can align with fire spread direction and cause rapid rate-of-spread increases.
Question 3: What information is NOT typically required when requesting a spot weather forecast from the National Weather Service?
- Elevation of the fire
- Fuel type at the location
- Names of firefighters on the incident (Correct answer)
- Aspect and slope of the terrain
Correct answer: Names of firefighters on the incident
Spot weather forecast requests require topographic and environmental data; individual firefighter names are not part of the meteorological information needed.
Question 4: A psychrometer is used to measure which weather variable critical to fire behavior?
- Wind speed and direction
- Relative humidity and dew point (Correct answer)
- Atmospheric pressure and elevation
- Temperature lapse rate
Correct answer: Relative humidity and dew point
A psychrometer uses wet-bulb and dry-bulb thermometers to determine relative humidity and dew point, key variables for assessing fuel moisture and fire danger.
Question 5: What is the significance of the 'critical fire weather pattern' involving an upper-level ridge of high pressure?
- Upper ridges bring moisture and suppress fire danger
- Upper ridges cause atmospheric subsidence, suppressing cloud formation and creating hot, dry, and increasingly windy conditions (Correct answer)
- Ridges always produce frontal passages that bring rain
- Upper ridges decrease surface wind speeds to calm conditions
Correct answer: Upper ridges cause atmospheric subsidence, suppressing cloud formation and creating hot, dry, and increasingly windy conditions
An upper-level ridge produces subsidence that warms and dries air as it descends, suppresses cloud cover, allows intense solar heating, and can enhance surface winds — a classic critical fire weather pattern.
Question 6: When calculating fire weather observations using the Fire Weather Index system, which parameter most directly reflects the dryness of fine fuels?
- Wind component
- Fine Fuel Moisture Code (FFMC) (Correct answer)
- Drought Code (DC)
- Initial Spread Index (ISI)
Correct answer: Fine Fuel Moisture Code (FFMC)
The Fine Fuel Moisture Code (FFMC) in the Fire Weather Index system reflects the moisture content of fast-drying fine fuels that respond to daily weather changes.
Question 7: A weather observation notes 'temperature 95°F, relative humidity 8%, wind 25 mph gusting to 40 mph.' What action should a Division Supervisor take?
- Continue normal operations since all three conditions must exceed thresholds simultaneously
- Monitor conditions but wait for an official Red Flag Warning before changing tactics
- Immediately evaluate crew positioning, escape routes, and safety zones given extreme fire weather (Correct answer)
- Request a spot forecast before taking any action
Correct answer: Immediately evaluate crew positioning, escape routes, and safety zones given extreme fire weather
The combination of high temperature, critically low humidity, and strong gusty winds represents extreme fire weather requiring immediate evaluation of crew safety positioning without delay.
S-130 Weather Observation 5
What is the 'mixing height' and how does it influence fire behavior?